Method of toughening steel



Patented Aug. 30, 1932 .I UNITE STATES PATENT OFFICE GEORGE A. JOHNSON, OF CHICAGO, ILLINOIS, ASSIGNOR TO W. H. MINER, INCL, OF CHI- OAGO, ILLINOIS, A CORPORATION OF DELAWARE iuE'rHon or TOUGHENING STEEL 1N0 Drawing. Application filed August 30, 1929, Serial No. 389,612. Renewed February 20, 1932.

thedesired degree of hardness in high car hon steel may be obtained by quenching by immersion in a cooling medium after the" article has been heated in a furnace, the steel being heated as nearly as possible to a temperature at which the highest degree of refinement and also the formation of the hard constituents is obtained so that when quenched the steel will posses these qualities, the toughening of low carbon steel castings by heretofore known similar processes has not been found commercially practical.

- -I n commercial practice and especially.

where mass production is'involved, when low carbon steel articles are treated in the usual manner by first heating the same for a predetermined time in a furnace and thenquickly cooling by quenching, many of the articles produced are defective and not toughened to the proper degree because the same are not quenched at precisely the correct heat. This is due to the factthat by the methods employed prior to my invention thequenching heat for a quantity of articles was mere ly approximated, the articles being quenched after being heated for a certain length of time in a furnace, the heat of which is maintained at the estimated proper fixed temperature for bringing the articles to the required quenching heat. In addition to the quenching temperature being inaccurately determined by following the furnace temperature as indicated by-the usual measuring device, namely: a 'pyrometer, which gives-a temperature reading difi'erent from that of the articles within the furnace, the quenching temperature further varies on account of the variations in loss of heat and conse:

quent change in temperature of the different articles while being handled in removal from the furnace and in conveying the same to the quenching bath. The quenching temperatures determined by the methods referred to are further inaccurate especially when such methods are employed in connection with mass production because no con- 'sideration is given to the fact that the critical quenching temperature will vary for different articles because of variations in the carbon content thereof, due to unavoidable manufacturing conditions.

The main object of my invention is to overcome the defects pointed out, by providing a method of toughening low carbonsteel wherein the quenching operation is performed at that instant corresponding exactly to the best condition as to refinement and tough constituents of each individual heated steel article.

A further object of the invention is to provide a method. of toughening low carbon steel by quenching, wherein the steel articles are heated to an extent in excess of the known critical temperature at which best refinement and toughening efiect are obtained, permittingthe sameto cool slowly, testing the same to determine its condition while cooling,, and quenching the same immediately when the critical temperature, while cooling,is indicated by the testin device.

Another object (if the invention is to pro vide a method of toughening low carbon steel articles, including the following steps: subj ecting the article to' heat to raise the same to a temperature known to be above that at which the steel loses its magnetic quality, removing the article from the influence of the heating medium, thereby permitting the temperature of. the article to slowly fall, testing the heated article for magnetic qualities while the temperature is slowly falling; and immediately upon the article showingv magnetic qualities quenching the same in a cooling medium. 7

Another object of the invention is to provide a commercially practical. efficient and economical method or process adapted par ticularly for mass production of toughened steel articles of relatively low carbon content wherein the articles are successively,-

removing the article from the furnace and suspending the same over a quenching bath, thereby removing the same fromthe influence of the heat of the furance and permitting slow cooling; testing the article while slowly cooling, for magnetic attraction by means of a permanent magnet; and immediately upon said article exhibiting attraction for the magnet dropping said article in the quenching bath.

Other objects of the invention will more clearly appear from the description and claims hereinafter following.

In disclosing my invention, I will describe the method as employed in toughening steel in connection with articles having a relatively low carbon content, such as cast steel cylinders employed in connection with shock absorbing devices of railway draft riggings. These cylinders are preferably manufactured of open hearth steel with a relatively low carbon content, varying from approximately 24% to 32%. I

A quantity of cast cylinders of the character described are placed in a furnace and subjectedto the heat thereof for a predetermined period of time. The furnace is maintained at a substantially constant temperature, the

usual pyrometers being employed in connec tion With the furnace to indicate the temperature thereof. The indicated heat is maintained at such a point that the articles in the furnace will be raised to a temperature above that at which the steel having the carbon content hereinbefore indicated reaches the highest degree of refinement and also the formation of the tough constituents occurs. This brings the steel to a point above the critical quenching temperature of the same. Above the critical quenching temperature of the steel cylinders having the low carbon content indicated, the steel loses its magnetic qualities while below said temperature it again becomes magnetic and this characteristic is taken advantage of in my improved process as will.

be more fully hereinafter pointed out.

' In low carbon steel of from, say, 24% to 32% carbon content,the temperature at which best results are obtained in quenching, and which temperature corresponds to what is known as the lower critical temperais changed producing a high degree of refine-' ment and toughening effect. Quenching at this lower critical temperature will, therefore, produce a tough steel possessing also the desired degree of refinement. When heated to a much greater degree, that is, considerably above the lower critical, the molecular structure is still further changed, the grain size being increased to such an extent that when the article is quenched brittle steel will be produced, which is obviously undesirable. It is evident, therefore, that it is important to heat the steel to an extent slightly above the lower critical range but not considerably beyond this point. In my improved method, I therefore, maintain the furnace at a proper temperature to produce this desired result.

It is a well known fact that low carbon steel having the percentage of carbon hereinbefore referred to becomes non-magnetic, for all practical purposes, when it is heated to a temperature above that corresponding to the lower critical and that it again becomes magnetic when the temperature is lowered, gradually increasing in magnetic qualities as the temperature continues to fall. It is further pointed out that as the low carbon steel changes from the non-magnetic state to magnetic state in cooling it also possesses the desired characteristics of refinement and for toughening effect, and that when quenched. I

at this temperature the best results are obtained. Inasmuch as the critical quenching temperature varies in accordance with the proportion of carbon content of the steel and the change in magnetism varies accordingly, this change in magnetic condition is a true and accurate indication of the proper quenchwithin the furnace.

vidual article at the proper temperature.

As hereinbefore pointed out, I heat the steel cylinders, in a furnace, to a point above the lower critical temperature for the particular low carbon steel employed. Inasmuch as in diiferent articles the carbon content of the steel may vary as much as .08% or more due to unavoidable manufacturing conditions, I maintain the furnace at a temperature relatively slightly above the lower critical temperature for 24% carbon content, there-by assuring that all articles of the same heat or different heats whether of 32% carbon content or .2% carbon content will be heated to a degree Where the magnetic quality is substantially lost or negligible. The heating operation is preferably continuous. and a number of steel cylinders are placed The cylinders are preferably arranged in a plurality of rows and are fed through the furnace continuously by a conveyor. As the cylinders are removed from one end of the furnace, the supply 1s replenished at the other end. Each cylinder is left in the furnace a suificient length of time, preferably about two hours, to assure that the same has been thoroughly heated throughout. The cylinders are toughened oneat a time, the same being removed from the furnace and suspended above a quenching bath. While thus suspended, the article is tested immediately to determine the molecular condition thereof while slowly cooling so that the quenching operation may be performed at precisely the proper temperature for the individual article. In order to test the molecular condition of the article, the temperature of the same may be determined prior to quenching by suitable means as it is well known that the temperature of a steel article having a definite or known carbon content is an indication of its molecular condition. I prefer to employ the magnetic'test to obtain an indication of the molecular condition of the. article and the proper quenching temperature thereof. While'the article is suspended as hereinbefore pointed out, the magnetic test is immediately applied,

the operator using a small sensitive magnet for this purpose, the same being preferably arranged at the end of a rod. The operator holds the magnet slightly spaced from the cylinder so that the same will be immediately attracted when the steel first becomes magnetic. The article is thus tested continuously until magnetic qualities are indicated. As the temperature of the cast steel.

cylinder'slowly falls due to exposure to the atmosphere, the steel will gradually change froma non-magnetic state to a magnetic state. As soon as the steel becomes even slightly magnetic, the same will attract the the proper quenching temperature has been reached. Immediately upon being given this indication, the operator drops the steel cylinder in the quenching bath, thus effect. ing quick cooling of the same at precisely the proper instant to produce the desired toughness. The described operation is repeated with the other cylinders successively.

As will be evident from the preceding description, I have provided a simple, efiicient and extremely accurate method for toughening low carbon steel articles which assures products of uniform strength, especially where quantity production is involved. This uniformity is assured by obtaining directly,

for each article, a true indication of the temperature immediately before quenching and while the temperature is falling, thereby eliminating defects and failures due to articles being quenched at varying temperatures, for example when the pyrometer reading of the interior of the furnace is taken as an indication of the proper quenching temperature and also when in quantity produc tion different articles vary in carbon content.

Inasmuch as my improved method entirely eliminates the danger of failure due to lac of uniformity of toughness and refinement of steel castings, thus eliminating the hazard of using castings which have not been toughened at the proper temperature and which are structurally weak if quenched above or below the proper temperature, the commercial use of toughened steel castings and mass production thereof is made practical, thereby obtaining a decided advantage over annealed or soft steel castings now generally employed, in that the toughened steel castings are at ings and have an ultimate tensile strength of approximately 100,000 pounds or overv per sider the preferred manner of carrying out 'least 50%' stronger than the annealed castmy invention, but the same is merely illustrative and I contemplate all changes and modifications that come within the scope o the claims appended hereto.

I claim:

1. A method of toughening steel, including the following steps: Heating the steel to a temperature above that corresponding to that at which it loses its magnetic qualities, removing the steel from the influence of the heating medium to permit slow cooling until it again exhibits magnetic qualities; and then immediately quenching the steel.

2. A method of toughening lowcarbon cast steel articles including the following steps:

Heating each article to an extent in excess of the temperature at which it loses its magnetic qualities; removing the same from the influence of the heating medium to permit slow cooling in the atmosphere; slowly cooling said article in the atmosphere until it reaches a temperature corr esponding so that at which it again becomes magnetic; and quenching the same immediately upon the last named temperature being reached.

3. A method of toughening steel including the following steps: Heating the steel to a temperature above at which it becomes nonmagnetic; removing the stcel from the influence of the heating medium to allow'slow cooling; testing the steel for magnetic qualities while slowly cooling; and, immediately upon said steel exhibiting magnetic qualities, quenching the same.

4. A method of toughening cast steel articles having a relatively low carbon content, including the following steps: Heating a quantity of steel articles in a furnace, the temperature of which is maintained above that necessary to heat the articles to a temperature at which the same become non-magnetic; removing the articles singly and successively from the furnace and suspending the same over a quenching bath, thereby removing the articles from the influence of the heat of the furnace and permitting slow cooling; testing the articles, successively, with a magnet while slowly cooling to determine their magnetic qualities; and immediately upon each article exhibiting attraction for said magnet dropping the same in the quenching bath to effect quick cooling and toughening thereof.

5. A method of toughening cast steel articles of varying low carbon content including the following steps: Heating the article in a furnace the heat of which is maintained at a predetermined fixed temperature above that necessary to heat the article to a temperature at which it becomes non-magnetic and subjecting the same to said temperature for a suflicient time to thoroughly heat the same throughout; removing the article from the furnace to permit slow cooling; testing the article for magnetic attraction by means of a magnet, while slowly cooling; and immediately upon magnetic qualities of said article being indicated by attraction for said magnet, immersing said article in a quenching bath to rapidly cool the same to effect toughening the: cot.

6. A method of toughening low carbon cast steel articles including the following steps: constantly feeding articles into a furnace, the temperature of which is above that at which the steel loses its magnetic qualities; removing said articles from the furnace successively, thereby effecting slow cooling thereof; constantly testing each'article while slowly cooling, bv means of a magnet; and quenching the same immediately upon the magnet being attracted thereby.

7. The herein described method of toughening a plurality of articles of low carbon steel, in which the carbon content in any one of said plurality of articles may vary anywhere from a known minimum to a known maximum, which includes: Heating all of a til the temperature thereof falls to the point I where the same again becomes magnetic; and thereupon immediately quenching each article while still at said last named temperature.

8. The herein described method of toughening a plurality of low carbon steel articles in which the carbon content in any one of said plurality of articles may vary anywhere from a known minimum to a known maximum, which includes: Heating all of said articles to such a temperature that the mag netic qualities of an article having the said minimum known carbon content will disappear; then cooling said articles; and, while cooling, testing each article individually until it first reacquires its magnetic characteristic and quenching each article immediately upon it reacquirmg its magnetic characteristic.

In witness that I claim the, foregoing I have hereunto subscribed my name this 26th day of August 1929.

' GEORGE A. JOHNSON. 

